A cellular wireless network such as the GSM network includes a plurality of ground base stations interconnected by the wired telephone network and which can be accessed by the mobile terminals within the radio cell of one station. A so-called up-link, that is from the terminal to the network, is implemented in a first frequency band whereas the down-link from the network to the terminal is implemented in a second frequency band. Each frequency band is divided into several channels, for example 62 channels, each of a given width, for instance 200 kHz. A spectrum gap called the “duplex spacing” separates the uplink frequency band from the down-frequency band.
Each base station must exhibit sufficient range that the cell under consideration spills over into its neighboring cells in order to eliminate any danger of communication loss when a mobile terminal changes cells. This requirement entails increasing the stations' transmitted power beyond the inherently required power.
Therefore a mobile wireless terminal effectively receives the transmissions from two stations when the wireless terminal is situated in the overlap zones of two cells. Actually the wireless terminal receives many more stations and, when powered 1 it communicates in a wireless manner with the electronically nearest station, by feeding the nearest station a radio level which is the highest from the six optimal levels. Within the overlap zone, the transmissions from the station not selected by the terminal constitute an interfering signal of the same approximate magnitude as the useful signal of the hookup cell. Also, because the signals from the two stations include frames having the same structure, the terminal may not be able to effectively filter the spectrum to attenuate the noise signal.
The operator is allocated a range of frequencies representing a costly resource with which to set up the radio network. When in this range, the operator regularly configures carrier frequencies, or transmission channels. The frequency gap between two carriers is sufficiently large so that the cross-channel interference remains less than a given threshold for good operation on account of the receivers' selectivities.
The number of the available carrier-frequencies however is much less than the number of channels required in the entire network. As a result the same frequency value must be used several times, and a check must be carried out in each cell to assure that the interference level at its carrier(s) and originating at the stations of the other neighboring cells remains below the threshold of proper operation. In other words, at any point in a cell, a safety margin must be provided between the received useful signal and the level of the interfering signal at the frequency under consideration. Because one network comprises several thousand stations that interfere with each other, calculating such interferences entails a large number of hours of calculation when using a conventional computer of average power.
This problem is compounded furthermore in that a terminal which is also linked to a station tolerates inversion between the relative signals received from its stations and from another station and links up with the latter only if the gap between the levels exceeds a given value of a hysteresis threshold. Again, the levels received from the diverse stations can randomly change in time as a function of the radio transmission and the position of the mobile terminal.
Moreover the noise signals from distant-station frames exhibit mean energies that change with traffic and the phases of their received carriers can slip relative to each other and in this manner can produce random modulation of the instantaneous amplitude of the received composite noise signal constituting their vector sum.
When the operator plans the network by fixing the positions and the radio features of the stations, the frequency allocation therefor must be based on optimization computations, the purpose of which is to determine a maximum number of occurrences to each carrier in the network while also assuring optimally the safety margin of the real signal in the light of the above hazards.
One of the factors defining the safety margin involves using constraint matrices listing constraints which must be obeyed between the various cells of the network.
It is known to use so-called “hard” constraint matrices for such planning. However, these matrices often are determined from the start on the basis of criteria assigned by the programmer and then are used in planning.
Illustratively the programmer in the prior art fixes his matrix by assigning a coefficient, for instance the value of 0, to the intersection of column Cj and the line Cl of the constraint matrix when the gap between the two channels of two cells Ci, Cj already is 0 (that is when the two cells are at the same frequency), or the value “1” when the gap between the respective frequencies has a first, fixed value and the value “2” when the gap between the respective frequencies has a second fixed value.
Such definitions however are frozen and subjective.
An objective of the present invention is to create an optimizing tool for creating constraint matrices that reflect reality as closely as possible and to use these optimized constraint matrices in planning a cellular wireless telephone network.